Murraya paniculata is a great tropical for bonsai. Good for "indoors for winter, outdoors for summer" bonsai. Summer repotting, while the tree is in active growth is the norm for tropicals. If you want to repot your Murraya at this time of year in Atlanta, the timing is good. You still have a couple months of warm weather during which the tree can recover. In the future repotting in early summer has the advantage of the whole growing season to recover, but you still have enough months left that repotting now is not an issue.
For Murraya, depending on what you want from the tree, would determine what media I would use. If you want the trunk to increase in diameter, you will need growth, foliage and branches to extend in order to increase the diameter of the trunk. In that case, I would consider moving it into a larger nursery style pot, or a low but wide pot, such as a bulb pan or an azalea pot. In such a case, I would use an organic mix, a potting soil even, to encourage a rapid growth rate.
If you are happy with the diameter of the trunk, and you want the size of the tree to be relatively static, then I would go to an inorganic soil.
So which type of media you choose depends on your purpose. Inorganic media, especially with broadleaf trees, tends to lend itself towards slower growth. For conifers, and desert dwelling trees, inorganic media is the first choice, but for broadleaf trees, the generally somewhat more dry nature of inorganic mixes definitely slows the growth of broadleaf trees, both tropicals and deciduous.
